Air pollution analysis and monitoring has become increasingly sophisticated as more is learned about the health effects of the specific components of air pollution and as analytical techniques improve to provide better analyte selectivity and lower detection levels.
Aerosols are one air pollution component receiving increased scrutiny. In particular, sulfur particulates, i.e., suspended particles, in air, that consist at least partially of sulfur, are of concern. This suspended material, whether or not it contains sulfur, is referred to as “PM” (particulate matter or particulate material) and is one component of combustion related aerosols, which are believed to contribute to health problems related to air pollution. Health-related effects may be most severe during periods of high concentration of these materials (PM events) in the atmosphere. Particles containing a high proportion of sulfur may be particularly harmful, and it is believed that the smaller particles may be of greatest concern. Data show that most sulfur-containing particles of less than about 1 micron in size are produced by stationary sources, such as fossil fuel power plants, and that particles larger than about 1 to 3 microns are mechanically introduced, rather than products of combustion.
The United States Environmental Protection Agency (EPA) has set standards at two different particle size thresholds, PM 2.5 (particles less than 2.5 microns in size) and PM 10 (particles less than 10 microns in size). Of particular interest, due to the health effects, is PM 2.5, for which a direct correlation with increased morbidity has been shown. As the sulfur content of these particles may be the greatest contributor to increased morbidity, the measurement of sulfate containing particles less than 2.5 microns in size is of particular interest.
As it is believed that the prevalent form of sulfur in particulate matter is sulfate, sulfate is of primary concern and analytical results may be reported in terms of either sulfur or sulfate.
Sulfate analysis presents a number of problems to the environmental analyst, including volatility, reactivity and stability. One current method of particulate sulfate analysis includes flame photometric detection (FPD), such as described in Appel, B R, R L Tanner, D F Adams, P K Dasgupta, K T Knapp, G L Kok, W R Pierson, K D Reiszner (1989); Method 713: “Semi-Continuous Determination of Atmospheric Particulate Sulfur, Sulfuric Acid, and Ammonium Sulfates” is described in Methods of Air Sampling and Analysis, 3rd edition, J P Lodge, Jr., editor, Lewis Publishers, Inc., Chelsea, Mich. This technique is a labor intensive method that requires specialized training as well as a source of hydrogen. Other methods include automated wet collection followed by ion chromatography (IC) analysis and single particle analysis using Time of Flight coupled with Mass Spectroscopy. Of particular use to those skilled in the art would be a method capable of accurately measuring particulate sulfate without requiring extensive user training or the use of explosive gasses or expensive instrumentation. In addition, to accurately characterize a PM event, a continuous measurement would provide more useful information than does a snapshot, or batch-type, analysis.